This invention relates to a motor controller and relates particularly to a motor controller for high speed, brushless DC motors to enable the motor speed to be controlled between minimum and maximum speeds with optimum efficiency.
International Patent Application No PCT/AU98/00035 describes a high speed, brushless DC motor which is designed particularly for driving a refrigerant compressor or the like. This motor has a rotor formed with a rare earth magnet (NdFeB) within a non-magnetic sleeve, and a stator winding of very low inductance to enable near unity power factor to be achieved between the base speed and maximum speed of the motor.
The present invention enables a motor of the type described in the aforesaid international patent application, or motors of similar design to be controlled without the need utilize complex and relatively expensive control equipment.
In order that the motor speed is able to be controlled within desirable limits using low cost control components, it is necessary to devise a motor controller which is able to take advantage of the low inductance design of the stator windings. Accordingly, it is desirable to provide a motor controller which is able to control the speed of a high speed, DC motor having low inductance stator windings.
It is also desirable to provide a motor controller of relatively low cost but effective in providing the necessary speed control required for a refrigeration compressor or the like.
It is also desirable to provide a motor controller which can be used in relation to motors of a range of sizes and structural make up.
It is also desirable to provide a motor controller which is able to control the motor speed in accordance with load demands.
According to one aspect of the invention there is provided a motor controller for a high speed, DC electric motor including sensors to detect the relative position of the rotor of the motor, speed calculator to receive position signals from the sensors and to calculate therefrom the approximate relative position and speed of rotation of the rotor, a vector rotator which receives actual speed and position signals from the speed calculator, an error signal generator to generate an error signal corresponding to any detected difference between a command speed signal and the actual speed signal, a proportional integrator to receive any error signal and produce a signal for the vector rotator corresponding to the desired vertical component of the voltage vector required to achieve the command speed, and power stage to amplify the vector rotator output to supply to the motor.
The controller of this invention is designed particularly for use in relation to high speed electric motors having a low inductance stator winding so as to maintain close to unity power factor during the range of operational speeds. Such an electric motor, as is described in the aforesaid International Patent Application No PCT/AU98/00035, is designed for use in a refrigeration system in which the speed of rotation of the centrifugal compressor is load-dependent. Refrigeration systems invariably run at less than full load capacity for a relatively large part of their operating time. The controller of the invention ensures that the power requirements of the motor result in maximum efficiencies over the total operation speed of the compressor. This is done by the controller generating a voltage vector in response to the sensors to maintain the motor current substantially in phase with the electric motive force (EMF) before the voltage limit is reached. Any further speed increase required after the voltage limit is reached is achieved by rotating the voltage vector while maintaining the amplitude constant.
In one preferred embodiment of the invention, the position sensors determine the rotational position of the rotor and its instantaneous rotational speed. The instantaneous speed determinations cannot detect acceleration between sensor positions so that the calculated actual speed of the rotor may include a degree of approximation. Any difference, however, between the detected speed and the actual speed is not significant for the purposes of the speed calculation.
The calculated actual speed is compared with a speed command signal to detect any difference. The speed command signal may be derived from a refrigeration system control circuit producing a speed signal corresponding to detected system load.
A vector rotator generates control voltages, the vertical and horizontal components of which reflect the speed command as modified by the difference with the detected speed.
Preferably, a conditional switch is provided in the controller so that the voltage amplitude and voltage angle are maintained commensurate with the command speed signal to maintain a minimum current for a given torque output. The conditional switch, in one position, applies a function of the actual rotor speed as the horizontal component and load demand as the vertical component to the vector rotator. The switch moves to a second position when the vertical and horizontal components of the voltage vector satisfy the condition xcexcq2+xcexcd2=1, and field weakening takes effect and any further increase in speed causes the vector rotator to rotate the voltage vector with the amplitude of the vector unchanged. In this way, the power factor is maintained close to unity.
The sensors which detect the relative position of the rotor comprise three position sensors displaced 120xc2x0 from each other. Because the sensors detect the instantaneous position of a magnetic field rotating with the rotor, and as the rotational speed may vary between sensors, the actual rotor position measured is necessarily approximate due to the possible speed changes between the sensor positions. However, at rotational speeds of between 20,000 rpm and 55,000 rpm, the approximation may be disregarded for the purposes of the controller of the invention.
In order that the invention is more readily understood, an embodiment thereof is described with reference to the accompanying drawing.